Telecommunication between facsimile apparatuses is generally performed in accordance with a half-duplex channel communication protocol governed by V.34 recommendation of ITU-T (International Telecommunication Union Telecommunication Standardization Sector). In the V.34 recommendation, a communication format is determined in phase 1. A symbol rate and a carrier frequency are determined in phase 2. In phase 3, an equalizer training of a primary channel is performed.
After phase 3, a control channel is started up. A signal speed of the primary channel is determined by exchanging modulation parameter sequence signals MPh. Next the primary channel is resynchronized. A source MODEM sends a S signal, a Sbar signal, a PP signal for synchronization and a sequence-B1 signal. Following them, the source MODEM sends image data.
The recipient MODEM receives signals from a start of a resynchronization to a completion of the reception of image information. When a plurality of document papers are to be sent, a series of steps (i.e., a resynchronization of a primary channel, a transmission of the image information and a resynchronization of the control channel) are repeated.
In the resynchronization of the control channel, when the documents are received correctly, a MCF (message confirmation) message is returned. When the received communication includes an error, a PPR (partial page response) message is returned so that the information which was not received correctly can be transmitted again. When the source MODEM receives the PPR signal, only the portion of the information containing an error is transmitted again.
When the source MODEM receives the MCF signal, the source MODEM transmits a next page if the next page is followed. If the next page is not followed, the source MODEM performs a disconnection operation by transmitting a DCN (disconnect) signal.
In the resynchronization of the primary channel, the recipient MODEM roughly adjusts a clock of the recipient MODEM to synchronize with a clock of the source MODEM by detecting the S signal and a transition from the S signal to the Sbar signal. Further, the recipient MODEM continues to synchronize using the PP signal. For this reason, the MODEM sequence can not proceed until detection of the S signal and the transition from the S signal to the Sbar signal is completed.
In the half-duplex channel communication, recovery procedures are provided up to phase 3 of the V.34 recommendation. However, a recovery procedure for the resynchronization of the primary channel is not provided. As a result, if an unintentional disturbance such as instant shutoff occurs while the recipient MODEM receives the S signal, the recipient MODEM is held in a waiting state until reception of the S signal is complete. Meanwhile, the source MODEM moves to a sequence of resynchronization of the control channel after transmission of the primary channel.
Thus, the source MODEM and the recipient MODEM can not proceed if communication line trouble (for example the instant shutoff while the source MODEM transmits S signal) occurs. The host CPU performs a disconnection process forced due to timeout, and the facsimile communication terminates.